Antenna and wireless tag

1. An antenna comprising: a conductor plate having a notch, the conductor plate being divided into a first region and a second region by the notch and having a power feed point at one end of the notch; and a capacitor that bridges the first region and the second region, wherein the notch is a ribbon-shaped notch, and a capacitance of the capacitor is set so as to be not less than 0.5×Co but not greater than 1.5×Co, where Co [F] is C obtained by substituting d=W into the following Equation (A): C = 1 120 ⁢ ⁢ π ⁢ ⁢ f ⁢ { ln ⁡ ( 4 ⁢ ⁢ h d ) - 1 } ⁢ tan ⁢ { 2 ⁢ ⁢ π λ ⁢ ( λ 4 - h ) } ⁡ [ F ] ( A ) wherein f [Hz] is a resonant frequency of the antenna, λ [m] is a resonant wavelength of the antenna found from λ=c/f where c is the speed of light, h [m] is a total length of the notch, and W [m] is a mean width of the notch.

2. The antenna according to claim 1 , wherein the first region and the second region of the conductor plate are provided on a single planar substrate.

3. The antenna according to claim 1 , wherein: the conductor plate is provided within a rectangular region of equal to or smaller than 85.6 mm×54.0 mm; and a resonant frequency of the antenna is equal to or lower than 322 MHz.

4. The antenna according to claim 1 , wherein: the notch is a U-shaped notch; and the conductor plate is divided, by the notch, into (i) the first region which is a rectangular region surrounded by the notch and (ii) the second region which is a U-shaped region surrounding the notch.

5. The antenna according to claim 4 , wherein the U-shaped notch includes three linear parts made up of two parallel linear parts and the other one linear part, the other one linear part being longer than each of the two parallel linear parts.

6. The antenna according to claim 1 , wherein at least part of the notch has a meander shape.

7. The antenna according to claim 1 , wherein a total length of the notch is less than λ/4 where λ is a resonant wavelength of the antenna.

8. The antenna according to claim 1 , wherein the first region and the second region are provided on an identical plane.

9. The antenna according to claim 2 , wherein one of the first and second regions is on a back surface of the planar substrate and the other is provided on a front surface of the planar substrate.

10. The antenna according to claim 2 , wherein: the planar substrate is a multilayer substrate; the first region of the conductor plate is provided on a first layer that is an inner layer or an outer layer of the planar substrate; and the second region of the conductor plate is provided on a second layer that is an inner layer or an outer layer of the planar substrate, the second layer being other than the first layer.

11. A wireless tag comprising an antenna recited in claim 1 .

12. A wireless tag according to claim 11 , further comprising a battery, wherein the conductor plate and the battery are provided within a rectangular region of equal to or smaller than 85.6 mm×54.0 mm such that they do not overlap each other.

13. The wireless tag according to claim 11 , which has a shape of a card.